1. Field of the Invention
This invention relates generally to methods of fabricating stabilized composite superconductors such as Nb.sub.3 Sn.
2. Description of Prior Art
In order to make stable superconductors for use in solenoids to generate magnetic fields of high intensity, the common practice consists in juxtapositioning the superconductors with a non-superconductive support of very good heat and electrical conductivity and to effect a close thermal contact therebetween. For making composite superconductors of intermetallic composition, crystallizing according to the A 15 structure, particularly such as Nb.sub.3 Sn, a niobium tubular sheath containing a copper-tin solid solution alloy is embedded in a copper matrix having high conductivity. After this structure is coreduced to the desired size, it is wire-drawn and subjected to a heat treatment, causing a diffusion-reaction for forming Nb.sub.3 Sn.
Disadvantages of the conventional methods are as follows. In the conventional core containing tin-copper alloys, more than 80 atomic percent tin is used, but the differences in mechanical properties between niobium and the tin-copper alloy are so great (for example, the hardness of niobium is Hv = 180 compared with Hv = 15 for the tin-copper alloy) that non-uniformities in the niobium sheath often cause breakage so that fabrication difficulties occur and the size of the final product is limited. As another example, in a tin-copper alloy containing about 7 to 80 atomic percent tin, the lack of ductility causes the alloy to turn into powder-like grains inside the niobium sheath and also leads to breakage. As a result tin-copper alloys containing more than 7 atomic percent tin are very difficult to fabricate. Hence, copper-tin alloys containing less than 7 atomic percent tin (bronze) are normally used to make Nb.sub.3 Sn superconductors by means of a solid-diffusion process.
Composites consisting of a copper-tin alloy containing less than 7 atomic percent tin can be reduced by cold working since these alloys have ductility to a certain extent. However, work-hardening of the alloy is so great that only about 50% reduction in area is possible in cold working. Hence in practice, the process of producing a usable conductor involves numerous repeated working and heat treatment stages.
Another difficulty of using a copper-tin alloy containing more than 25 atomic percent tin is in the reaction involving tin and niobium to form Nb.sub.3 Sn. The Nb.sub.3 Sn formed from the reaction between the copper-tin alloy and niobium has layers of irregularities, including some Nb.sub.3 Sn islands dissolved in the copper-tin alloy, and contains discontinuities along the length of the product since the copper-tin alloy takes the liquid state at the reaction temperature required to form Nb.sub.3 Sn (700.degree. - 900.degree. C).